The present invention relates to an air separation plant and to a method of fabrication in which a main heat exchanger is designed to function in air expansion, waste expansion and nitrogen expansion plants. Even more particularly, the present invention relates to such an air separation plant and method in which distillation is conducted by mass transfer elements such as packings that are contained within prefabricated column elements.
Air is separated into its components within air separation plants having a main heat exchanger to cool compressed and purified air to a temperature suitable for its distillation and an air separation unit formed by single and double distillation column arrangements. In a double column air separation unit, higher and lower pressure columns are operatively associated with one another in a heat transfer relationship by a condenser-reboiler. The air is distilled in the higher pressure column to separate nitrogen from the air and to produce an oxygen enriched liquid column bottoms which is further refined in the lower pressure column to produce nitrogen and oxygen products. In single column air separation units, the air is separated to produce a nitrogen product as a tower overhead. The liquid column bottoms, enriched in oxygen, is expanded to a low pressure (and therefore a low temperature) to serve as a coolant to condense reflux.
In any air separation plant functioning at cryogenic temperatures there are warm end heat losses and heat leakage into the plant. In order to compensate for such effects, refrigeration can be provided by either expanding part of the incoming air, a waste stream, or part of a product stream. Such expansion produces a refrigerant stream. In case of air expansion, such refrigerant stream is typically introduced in a column. Refrigerant streams originating from waste and product expansions are fed into the main heat exchanger and then fully warmed.
The fabrication of air separation plants involves to a large extent the custom design and construction of its components. For instance, in order for the main heat exchanger to function in an air expansion plant, an intermediate outlet for the air must be provided so that the air is partially cooled prior to its introduction into the expansion engine. In a plant that function with waste or product expansion, such intermediate outlet for the air might not be needed. However, an intermediate outlet for the waste or product stream to be expanded must be provided so that the particular process stream to be expanded is partly warmed. Columns are also custom built, precision components that must have a sufficient height to provide the number of stages of separation that are needed for the particular distillation involved. All of such custom design and construction adds to the capital costs involved in the fabrication of the air separation plant.
As will be discussed, the present invention provides an air separation plant that can be constructed with prefabricated components to eliminate the added capital cost involved in custom design and construction of the components making up the air separation plant.